A power converter is a power processing circuit that converts an input voltage or current source waveform into a specified output voltage or current waveform. In a switch-mode converter, a pulse-width-modulation (PWM) controller is often used to control the on and off periods of switches employed therein. The controllable switches, in the switch-mode converter, are each required to conduct the substantial load dependent currents during the respective switching cycles. As a result, the switches are subjected to high switching stresses and high switching power losses that increase linearly with the switching frequency of the converter, thereby limiting the switches' usefulness at high frequencies. It is well known that, to increase the power density of the switch-mode converters and to reduce converter size and weight, the switching frequency may be increased. To realize the higher frequencies, the limitations associated with higher frequencies described above must be addressed. These limitations can be reduced if each switch in the converter changes its state (from on to off or vice versa) when the voltage across the switch or the current through the switch is zero at the switching instant. Converter topologies that reduce switching losses by turning the switches on or off when the current or voltage is zero are described as zero-current switching (ZCS) converters or zero-voltage switching (ZVS) converters, respectively.
An analysis of a ZVS half-bridge converter with PWM control is described, for example, in a paper entitled "Static and Dynamic Analysis of Zero-Voltage-Switched Half-Bridge Converter with PWM Control" by Tamotsu Ninomiya, et al., Proceedings of IEEE PESC '91, pp. 230-237 (1992); see also "Static and Dynamic Analysis of ZVS-PWM Half Bridge Converter" by T. Ninomiya, et al., IEE of Japan, MAG-90-82, August 1990, both of which are incorporated herein by reference. Ninomiya, et al., analyzed a half-bridge converter with an asymmetrical PWM control scheme and demonstrated quantitatively the improvement of the control characteristics performed by the asymmetrical regulation of a pair of switches employed therein.
Asymmetrical switching occurs when the two switches in a half-bridge converter are turned on and off complementarily. In the conventional switch-mode half-bridge converters, the power switches are typically maintained in a conducting state for an equal duration during each half of the switching period. For one duty cycle, alternately one switch, then the other switch, conducts during successive half periods of the duty cycle for the same length of time with a corresponding dead time between each conduction time period. Asymmetrical half-bridge converters differ from the conventional switch-mode half-bridge converters in that the switches conduct for unequal lengths of time with only a small deadtime between the alternating conduction periods.
A half-bridge converter employing an asymmetrical duty cycle to control the converter's switches is described in, for example, U.S. Pat. No. 5,245,520, issued on Oct. 10, 1993, to Imbertson, entitled "Asymmetrical Duty Cycle Power Converter," which is incorporated herein by reference. Imbertson describes using the small deadtime between conducting periods in an asymmetrical converter circuit to obtain low loss commutation of the power switches and using the variable conduction periods to regulate the output. Not only does the asymmetrical half-bridge converter provide ZVS capability, but due to the reduced voltage or current stresses on the switches the converter is suitable for high frequency applications.
An asymmetrical half-bridge converter, however, may suffer from having a DC bias current in the converter's power transformer and possibly a large ripple current component in the output of the converter when required to operate over a range of input voltages. The output ripple current can be reduced or eliminated by the design of the magnetics, however, at the expense of increasing the DC bias current in the converter's transformer. The DC bias current often necessitates that the magnetic core of the power transformer be increased to prevent the transformer from saturating. Therefore, care must be taken when determining the size of the transformer to take into account the largest DC bias current that may be present. This inevitably results in an increase in the overall size, weight and cost of the converter.
Accordingly, what is needed in the art is an improved converter that mitigates the above-identified problems and, more particularly, there is a need for an improved converter with a reduced or substantially zero DC bias current in the power transformer of the converter.